The goal of this proposal is to better understand the mechanisms by which autoreactive B cells get activated, differentiate into antigen producing cells, and participate into the autoimmune response by activating autoreactive T cells. We will use the NZM2410-derived B6.NZMSle1/Sle2/Sle3 lupus-prone strain (B6.TC), which constitutes an ideal model to analyze the immune defects responsible for lupus pathogenesis because of its simplified genetics (only ~ 6% of the NZM2410 genome). This allows co-cultures and adoptive transfer experiments between B6 and B6.TC to test the contribution of a single cell compartment in the context of a true genetic control, which has not been achieved in other lupus experimental models. We will use immunoglobulin HC transgenes, rheumatoid factor (RF) AM14, anti-DNA 56R, and anti-phosphorycholine (PC) M167 as a control, to track the fate of autoreactive B cells in the B6.TC model. Our primary focus on the AM14 model stems from its emerging significance as a generalized model for anti-nuclear autoreactivity in which the presence of the autoantigen can be controlled. To achieve these goals, we have two specific aims: 1. To define the contribution of the marginal zone B cells to autoimmunity in the B6.TC model. In vitro and in vivo experiments will define the mechanisms by which B6.TC B cells are selected to the MZ subset and participate in autoimmune pathology, and will establish whether there is a causal relationship between breach in MZB cell follicular exclusion and production of autoantibodies. 2. To define the functional mechanisms responsible for the loss of tolerance of RF AM14 B cells in the B6.TC model. B6.TC RF AM14 B cells are activated and differentiate into antibody producing cells by mechanisms that differ to what has been described in Fas-deficient strains, including a much greater role for polyclonal activation. A detailed analysis of these mechanisms will take advantage of our congenic system to systematically define the role of various cell compartments along the entire B cell developmental process. Overall, these results will provide a better understanding of how autoreactive B cells develop and contribute to autoimmune pathogenesis, and should also provide strategies to better target these cells.